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   难度：Easy
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   <h1 class="question_title">
    171. Excel Sheet Column Number
   </h1>
   <p>
    Given a column title as appear in an Excel sheet, return its corresponding column number.
   </p>
   <p>
    For example:
   </p>
   <pre>
    A -&gt; 1
    B -&gt; 2
    C -&gt; 3
    ...
    Z -&gt; 26
    AA -&gt; 27
    AB -&gt; 28 
    ...
</pre>
   <p>
    <strong>
     Example 1:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> "A"
<strong>Output:</strong> 1
</pre>
   <p>
    <strong>
     Example 2:
    </strong>
   </p>
   <pre>
<strong>Input: </strong>"AB"
<strong>Output:</strong> 28
</pre>
   <p>
    <strong>
     Example 3:
    </strong>
   </p>
   <pre>
<strong>Input: </strong>"ZY"
<strong>Output:</strong> 701
</pre>
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   <h1 class="question_title">
    171. Excel表列序号
   </h1>
   <p>
    给定一个Excel表格中的列名称，返回其相应的列序号。
   </p>
   <p>
    例如，
   </p>
   <pre>    A -&gt; 1
    B -&gt; 2
    C -&gt; 3
    ...
    Z -&gt; 26
    AA -&gt; 27
    AB -&gt; 28 
    ...
</pre>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre><strong>输入:</strong> "A"
<strong>输出:</strong> 1
</pre>
   <p>
    <strong>
     示例&nbsp;2:
    </strong>
   </p>
   <pre><strong>输入: </strong>"AB"
<strong>输出:</strong> 28
</pre>
   <p>
    <strong>
     示例&nbsp;3:
    </strong>
   </p>
   <pre><strong>输入: </strong>"ZY"
<strong>输出:</strong> 701</pre>
   <p>
    <strong>
     致谢：
    </strong>
    <br>
    特别感谢&nbsp;
    <a href="http://leetcode.com/discuss/user/ts">
     @ts
    </a>
    &nbsp;添加此问题并创建所有测试用例。
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